So strong pigments yellow urethane pigments for use with liquid urethane rubber urethane plastics eg fast cast resin or urethane foam do not use with silicone rubbers a single drop will go a very long way so start with small percentages of pigment because you can always add more when using smooth cast add the pigment to one component and mix before adding the other componentmake sure you wear latex gloves to prevent stains available in 2oz 57g bottles in a range of colours